Evolutionary associations of bill morphology, migration, and feeding ecology in Tyrant flycatchers (Tyrannidae)

Migratory birds experience changes throughout their annual cycle in food availability and diet, which may influence the evolution of feeding morphology. We investigated the evolution of bill morphology in relation to migratory phenotype, diet, and foraging behavior across 386 species of Tyrant flycatchers (Tyrannidae), and among migratory and non-migratory populations of the Fork-tailed Flycatcher (Tyrannus savana). At the interspecific level, bill morphospace extensively overlapped between migrants and non-migrants, with small differences in bill shape but not size. Migrants had deeper and narrower bills at inter- and intraspecific levels. However, this shape difference was not more pronounced between sister species differing in migratory phenotype, suggesting that the changes in migratory phenotypes during speciation were not associated with bill divergence. At the intraspecific level, migratory Fork-tailed Flycatchers also had smaller bills varying according to age but not sex. Across Tyrannidae, species with an invertivore-herbivore diet that do not forage on the ground and understory exhibited wider and shorter bills. Overall, our results suggest a weak but significant association between bill morphology and migratory phenotypes, with additional variation in diet, foraging strata, and age. However, bill morphology appears largely independent of migratory behavior, underscoring a decoupling of migration from foraging‑related morphological constraints.